1. Field of the Invention
This invention relates to apparatus for and a method of cooling a flow of molten material. More especially, the invention concerns the removal of heat from a flow of molten material, e.g. liquid metal, passing along or through a heat transfer conduit.
2. Description of the Prior Art
Heat transfer conduits in the form of hollow tubes or open-topped channels are known for removing heat from molten materials which are about to be cast. Conventionally, molten material to be cast is held at a temperature in excess of the liquidus temperature of the material in order to avoid premature solidification of the material within a transfer vessel before entry into a casting mould. As a result, the solidification process tends to relatively slow because of the significant amount of super heat and latent heat which has to be removed from the material in the casting process leading to relatively low throughput rates and consequent macro-segregation in the cast product.
Such a heat transfer conduit or carrier is disclosed in our United Kingdom patent 21117687B.
Known heat transfer conduits or carriers (hereinafter referred to simply as heat transfer conduits) are of tubular or channel-shaped construction and rely upon their mass or are cooled externally by water sprays, coolant jackets or fluidised beds to provide the required heat transfer. It is also known that by providing sufficient shear rate within a molten material most or all of its superheat and some of its latent heat can be extracted whilst still preserving a low viscosity within the molten material to achieve steady state heat removal without blockage of the conduit occuring.
When using a heat transfer conduit a shell of solidified material may form at the internal surface of the conduit, particularly so if the heat extraction rates are high. The formation of the shell can lead to certain problems which the present invention sets out to alleviate.
Typically, when a molten material is passed through a heat transfer conduit of high conductivity, the following sequence of events occurs:
(a) the liquid material solidifies as it comes into contact with the relatively cool internal surface of the conduit wall to form a solid shell, the equilibrium thickness and shape of which depends on the heat transfer conditions existing within the conduit, the condition of the molten material and the characteristics of the conduit;
(b) the thickness of the solidified shell increases to achieve an equilibrium temperature profile in which the radially outer surface temperature of the shell is considerably cooler than that extant at the radially inner surface of the shell. Thus, the shell tends to shrink and become separated from the conduit surface at various locations about its circumference;
(c) the heat transfer may ultimately be limited by the shell thickness and its resistance to heat transfer;
(d) the inner surface of the conduit increases in temperature and expands thereby promoting the formation of a gap between the opposed surfaces of the conduit and the shell.
Following use of known heat transfer conduits, a shell of solidified material remains within the conduit. The shell may be distorted thereby making it difficult to remove from the conduit. Hitherto, attempts to remove the shell from a hollow heat transfer conduit have resulted in damage to the conduit and/or ancillary equipment. It is preferable that the shell be removed prior to restarting the casting process since the presence of a solid shell at a relatively low temperature in a hollow conduit might result in blockage. The present invention sets out to provide a heat transfer conduit which overcomes the aforementioned problems and which also provides enhanced heat transfer characteristics between the conduit and molten material flowing therethrough.